The present invention relates generally to an offshore platform for a hydrocarbon production facility, and more particularly to a rigid jacket of the offshore platform which has a quadrapod structure.
Offshore production of hydrocarbons is typically enabled by placement of a platform at a desired offshore site. The platform comprises a jacket and a deck supported by the jacket, which houses a hydrocarbon production facility. The height of the upright jacket is greater than the depth of the water at the offshore site of the platform so that when the base of the jacket is positioned on the sea floor, the top of the jacket extends above the sea surface. The deck is mounted atop the jacket so that the deck is likewise positioned above the sea surface. Most offshore platforms are bottom-founded structures, which are fixed to the sea floor by attaching pilings driven into the sea floor to the base of the jacket.
There are a number of criteria, which must be considered in designing the jacket of a bottom-founded offshore platform. In particular, the jacket must be designed with sufficient strength to resist collapse or bending during periods of high loads or stress. For example, the jacket incurs relatively high loads or stress during initial placement of the jacket at the offshore site and during prolonged continuous in-place operation of the offshore platform in the sea environment, where the jacket is exposed to a broad range of wave forces. Waves are characterized by their height and period. Very large or tall waves are long period waves because of the physical limits on the steepness of the waves, while small or short waves are short period waves. For example, a classical Gulf of Mexico hurricane wave is 75 feet high and has a 12 second period. Smaller waves have correspondingly shorter heights and shorter periods.
Bottom-founded jackets are characterized as either xe2x80x9crigidxe2x80x9d or xe2x80x9ccompliantxe2x80x9d. A rigid jacket is substantially stiffer along its length than a compliant jacket. A rigid jacket is designed and constructed with sufficient strength to prevent the platform from being pushed over by very large waves. The natural period of sway for a rigid jacket is usually less than 3 seconds and the natural period of whipping is significantly shorter. The short natural period of the jacket avoids a resonant sway response to more numerous small waves, which could damage the platform through fatigue.
A compliant jacket, such as disclosed in U.S. Pat. No. 5,480,265, is not strong enough to directly resist large waves. Instead, a compliant jacket relies on substantial inertia and a very long sway period on the order of about 45 seconds to prevent the platform from being pushed over by very large waves. Thus, even very large waves with 12 to 16 second periods pass through the jacket too quickly for the jacket to respond to the wave. However, the fundamental requirement of a long sway period limits the utility of compliant jackets to relatively deep water on the order of about 1000 feet or more. The critical fatigue issue for a compliant jacket having a long sway period is the whipping period, which causes the jacket undue fatigue if it is too great. Therefore, it is necessary to design and construct a compliant jacket having a sufficiently long sway period, yet also having a whipping period within acceptable limits, to avoid failure of the jacket structure from wave induced fatigue. Although compliant jackets can frequently be constructed at a lower cost than rigid jackets due to reduced material requirements, design of compliant jackets can oftentimes be more complex than rigid jackets due to the difficulty in achieving both an optimal sway period and an optimal whipping period.
As noted above, the jacket is subjected to relatively high loads and stresses both during placement of the jacket and during in-place operation of the offshore platform. These loads and stresses have different characteristics, which must be considered in the design of the jacket. Most jackets are fabricated at onshore locations. After fabrication, the jacket is loaded onto a transport barge with the jacket lying on its side in a horizontal orientation. The jacket is transported atop the transport barge to a desired offshore site for placement. Placement of the horizontally oriented jacket on the sea floor is effected by one of two methods, either lifting or launching. The lifting method employs a heavy-lift vessel positioned alongside the transport barge, which engages the horizontal jacket, raises the jacket off the deck of the barge, reorients the jacket to an upright vertical position, and sets the jacket down in a vertical orientation on the sea floor at the desired offshore site. The launching method tilts the deck of the transport barge so that the horizontal jacket, which has a plurality of interior buoyancy compartments, slides laterally under the force of gravity along the deck into the sea and floats in a substantially horizontal orientation on the sea surface. The jacket interior is then flooded in a controlled manner, upending the jacket to a vertical orientation. A derrick barge positions the jacket over the desired offshore site and the jacket interior is flooded further, setting the jacket down on the sea floor.
A jacket, which is placed by the launch method, is preferably provided with a plurality of launch runners to facilitate sliding the jacket off of the barge. The launch runners protrude from the external framing of the jacket and are typically fortified by a separate launch box, which is added to the framing of the jacket and strengthens the jacket structure against the severe loads and stresses encountered during launch of the jacket.
There are any number of rigid jacket configurations known in the prior art. Nevertheless, it is commonly recognized that the most efficient configuration of a rigid jacket is a tripod configuration having three convergent legs. By most efficient configuration, it is meant that the tripod configuration achieves the greatest strength with the lowest material requirements, at least with respect to in-place operation of the platform. However, a rigid jacket having the tripod configuration lacks two parallel legs, to which launch runners can be mounted for launching the jacket. Therefore, placement of a rigid tripod jacket must be performed by the lift method. This undesirably limits the maximum design weight of the jacket to the capacity of the available heavy-lift vessel, which in turn limits the water depth and topsides load capacity for which the jacket can be designed.
The present invention recognizes a need for a launchable jacket having the desired in-place performance characteristics of a rigid jacket, yet having substantially reduced material requirements relative to known launchable rigid jacket designs. Accordingly, it is an object of the present invention to provide a launchable rigid jacket, which has sufficient strength to resist damage or failure caused by launching the jacket from a barge at a desired offshore site. It is another object of the present invention to provide such a launchable rigid jacket, which has sufficient strength to resist damage or failure caused by wave forces during in-place operation of the jacket at the offshore site. It is still another object of the present invention to provide such a launchable rigid jacket, which has reduced material requirements for its construction. These objects and others are accomplished in accordance with the invention described hereafter.
The present invention is a launchable rigid jacket of an offshore platform. The jacket is characterized by a base, a top and a height extending from the base to the top. The jacket has a structure including an exterior framing and a plurality of cross-sectional and intermediate frame sections. The exterior framing comprises four outside legs, each extending at least a majority of the height of the jacket. A first pair of the four outside legs is more closely spaced apart and a second pair of the four outside legs is more widely spaced apart to define four corners of a trapezoidal cross section of the exterior framing.
The cross-sectional frame sections are horizontally positioned at periodic intervals along the height of the jacket between the base and the top and the intermediate frame sections are horizontally positioned between adjacent pairs of the cross-sectional frame sections. Each of the cross-sectional frame sections has a front member, a rear member, a first lateral member and a second lateral member, which interconnect the four outside legs. The front, rear, first lateral, and second lateral members are part of the exterior framing and, in association with the four outside legs, provide each cross-sectional frame section with a cross section corresponding to the trapezoidal cross section of the exterior framing. The exterior framing in total defines an integral launch box for the jacket.
Each cross-sectional frame section encloses a cross-sectional conductor opening having at least one substantially perpendicularly oriented conductor passing therethrough from the base to the top. A cross-sectional conductor guide is positioned in each cross-sectional conductor opening and is connected to the cross-sectional frame section. The cross-sectional conductor guide slidably engages the conductor to laterally support the conductor, thereby permitting independent vertical movement of the conductor relative to the cross-sectional frame section while substantially preventing independent horizontal movement of the conductor relative to the cross-sectional frame section.
Each intermediate frame section only partially extends across the trapezoidal cross section of the exterior framing. The intermediate frame section encloses an intermediate conductor opening having the conductor passing therethrough. An intermediate conductor guide is positioned in the intermediate conductor opening and is connected to the intermediate frame section. The intermediate conductor guide slidably engages the conductor to laterally support the conductor, thereby permitting independent vertical movement of the conductor relative to the intermediate frame section while substantially preventing independent horizontal movement of the conductor relative to the intermediate frame section.
The jacket further comprises a first launch runner and a second launch runner. The first launch runner is mounted on one of the first pair of outside legs, which are more closely spaced apart, and the second launch runner is mounted on the other of the first pair of outside legs. The first pair of outside legs has a substantially parallel orientation to one another where the first and second launch runners are mounted.
The present invention is also a method for placing a launchable rigid jacket at an offshore site in a sea environment having a sea floor and a sea surface. The method comprises providing a jacket as described above and positioning the jacket on a barge deck of a transport barge in a horizontal orientation with the first and second launch runners engaging the barge deck. In particular, the first launch runner engages a first skidway formed in the barge deck and the second launch runner engages a second skidway formed in the barge deck. The jacket is transported on the transport barge to the offshore site and launched from the transport barge at the offshore site by sliding the first and second launch runners along the first and second skidways in the barge deck. The first and second launch runners are in continuous contact with the first and second skidways until the jacket departs the barge deck. The integral launch box bears launch loads on the jacket to prevent structural failure of the jacket during launching. Upon completion of launching, the jacket is upended, positioned and fixed to the sea floor with the base proximal to the sea floor and the top proximal to the sea surface. An above-water deck is then mounted to the top and is rigidly supported by the jacket.
The present invention will be further understood from the drawings and the following detailed description.